NCERT Solutions For Class 10 Maths Chapter 3

NCERT Solutions Class 10 Maths Linear Equations in Two Variables

Maths is a subject which requires students understanding and reasoning skills. Along with this, it also requires students to practice maths in a regular manner. Students of Class 10 are suggested to solve NCERT questions in order to practice the questions that are usually asked in the examination. From all the chapters, NCERT Solution for Class 10 Maths Linear Equations in Two Variables forms an important topic for the students, and one needs to practice it to score well in the examination. BYJU’S provide NCERT Solution for Class 10 Maths Chapter 3 Linear Equations in Two Variables in the pdf format. These solution are provided in detailed manner, where one can find step-by-step solution to all the questions of NCERT Solutions for class 9 maths chapter 3.

NCERT solutions for class 10 maths chapter 3 pdf are prepared by our subject experts under the guidelines of NCERT to assists students in their board exam preparations.

NCERT Solutions Class 10 Maths Chapter 3 Exercises

Exercise 1 

Question 1:

Astitva tells his daughter, “Seven years ago I was seven times as old as you were then and also three years from now, I shall be three times as old as you will be.”  Represent this situation algebraically and graphically.

Solution:

Let the present age of Astitva be ‘x’.

And, the present age of his daughter be ‘y’.

Seven years ago,

Age of Astitva = x-7

Age of his daughter =y-7

According to the question,

x7=7(y7) x7=7y49

x7y=42         ………………………(i)

 

Three years from now,

Age of Astitva will be ‘x+3’

Age of his daughter will be ‘y+3’

According to the question,

x+3=3(y+3) x+3=3y+9

x3y=6       …………………(ii)

 

Subtracting equation (i) from equation (ii) we have

(x3y)(x7y)=6(42) 3y+7y=6+42 4y=48 y=12

 

The algebraic equation is represented by

x7y=42 x3y=6

For x7y=42

x=42+7y

 

The solution table is

 

 

X -7 0 7
Y 5 6 7

 

For  x3y=6   or     x=6+3y

The solution table is

 

X 6 4 0
Y 0 -1 -2

 

The graphical representation is-

1

 

Question 2:

A cricket team coach purchases 3 bats and 3 balls for Rs 3900. Later on, he buys 1 bat and 2 more balls of the same kind for Rs 1200. Represent the situation algebraically and geometrically.

Solution:

Let us assume that the cost of a bat be ‘Rs x’

And,the cost of a ball be ‘Rs y’

According to the question, the algebraic representation is

 

3x+6y=3900 x+2y=1300

For 3x+6y=3900

x=39006y3

 

The solution table is

 

x 300 100 -100
y 500 600 700

 

For,

x+2y=1300 x=13002y

 

The solution table is

 

x 300 100 -100
y 500 600 700

 

The graphical representation is as follows.

2

 

Question 3:

The cost of 2 kg of apples and 1 kg of grapes on a day was found to be Rs 160. After a month, the cost of 4 kg of apples and 2 kg of grapes is Rs 300. Represent the given situation algebraically and geometrically.

Solution:

Let the cost of 1 kg of apples be ‘Rs x’

And, cost of 1 kg of grapes be ‘Rs y’

According to the question, the algebraic representation is

2x+y=160 4x+2y=300

For,  2x+y=160

y=1602x

 

The solution table is

 

X 50 60 70
Y 60 40 20

 

For 4x+2y=300,

y=3004x2

 

The solution table is

 

x 70 80 75
y 10 -10 0

The graphical representation is as follows.

3

 

Exercise-2

 

Question 1: 

Find the graphical solution for the given problems.

 (a).In Class 10th, 10 students participated in a Maths test. If there are 4 more girls than the total number of boys, find how many girls and boys participated in the test.

(b)The total cost of 5 erasers and 7 chocolates is Rs.50, but the total cost of 7 erasers and 5 chocolates is  Rs. 46. Now, calculate the cost of one eraser and the cost of one chocolate.

Solution:

(a)Let there are x number of girls and y number of boys. As per  the given question,  it is represented as follows.

x+y=10xy=4Nowx+y=10,x=10y

 

X 5 4 6
Y 5 6 4

 

Forxy=4,x=4+y
X 4 5 3
Y 0 1 -1

 

Hence, the table is represented in graphical form as follows.

e3.1 q1

From the figure, it is seen that the given lines cross each other at point (7, 3). Therefore, there are 7 girls and 3 boys in the class .

(b)Let 1 pencil costs Rs.x and 1 pen costs Rs.y. According to the question, the algebraic representation is

5x+7y=507x+5y=46For5x+7y=50, x=507y5

x 3 10 -4
y 5 0 10

 

7x+5y=46

 

X 8 3 -2
Y -2 5 12

 

Hence, the graphic representation is as follows.

e3.2 1.1

From the figure, it is seen that the given  lines cross each other at point (3, 5).

So, the cost of a eraser is 3/- and cost of a chocolate is 5/-.

 

Question 2:

Comparing the given ratios, a1a2,b1b2andc1c2,Figure out whether the lines are parallel or coincident:

(a) 5x4y+8=0

7x+6y9=0

(b) 9x+3y+12=018x+6y+24=0

(c) 6x3y+10=02xy+9=0

Solution:

(a) 5x4y+8=0

7x+6y9=0

Comparing these equations with a1x+b1y+c1=0

and a2x+b2y+c2=0

We get,

a1=5,b1=4,c1=8a2=7,b2=6,c2=9 a1a2=57 b1b2=46=23

Since a1a2b1b2

So, the pairs of equations given in the question have a unique solution and the lines cross each other at exactly one point.

 

(b) 9x+3y+12=018x+6y+24=0

Comparing these equations with a1x+b1y+c1=0

and a2x+b2y+c2=0

We get,

a1=9,b1=3,c1=12a2=18,b2=6,c2=24 a1a2=918=12 b1b2=36=12 c1c2=1224=12

 

Since a1a2=b1b2=c1c2

So, the pairs of equations given in the question have infinite possible solutions and the lines are coincident.

 

(c) 6x3y+10=02xy+9=0

Solution: Comparing these equations with a1x+b1y+c1=0

and a2x+b2y+c2=0

We get,

a1=6,b1=3,c1=10a2=2,b2=1,c2=9 a1a2=62=31 b1b2=31=31 c1c2=109

 

 

Since a1a2=b1b2c1c2

So, the pairs of equations given in the question are parallel to each other and the lines never intersect each other at any point and there is no possible solution for the given pair of equations.

 

Question 3:

Compare the given ratios, a1a2,b1b2andc1c2 find out whether the following pair of linear equations are consistent, or inconsistent.

(a) 3x+2y=5;2x3y=7

(b) 2x3y=8;4x6y=9

(c) 32x+53y=7;9x10y=14

Solution:

(a) 3x+2y=5;2x3y=7

a1a2=32 b1b2=23 c1c2=57

 

Since a1a2b1b2

So, the given equations intersect each other at one point and they have only one possible solution. The equations are consistent.

 

(b) 2x3y=8;4x6y=9

a1a2=24 b1b2=36 c1c2=89

 

Since a1a2=b1b2c1c2

So, the equations are parallel to each other  and they have no possible solution. So, the equations are inconsistent.

 

(c) 32x+53y=7;9x10y=14

a1a2=329=16 b1b2= frac5310=16 c1c2=714

 

Since a1a2b1b2

So, the equations are intersecting  each other at one point and they have only one possible solution. So, the equations are consistent.

 

Question 4:

Find whether the following pairs of linear equations are consistent or  inconsistent. If consistent, find the solution graphically:

(a)x + y = 5, 2x + 2y = 10

(b)2x + y – 6 = 0,4x – 2y – 4 = 0

Solution:

(a)x + y = 5, 2x + 2y = 10

a1a2=12 b1b2=12 c1c2=510=12

 

Since a1a2=b1b2=c1c2

∴ the equations are coincident  and they have infinite number of possible solutions.

So, the equations are consistent.

x + y = 5, 2x + 2y = 10

x 4 3 2
y 1 2 3

 

And, 2x + 2y = 10

x=102y2
x 4 3 2
y 1 2 3

 

So, the equations are represented as follows:

e3.2q 4

From the figure, it is seen that the lines are overlapping each other.

Therefore, the equations have infinite possible solutions.

 

Question 5:

Consider the two equations x − y + 1 = 0 and 3x + 2y − 12 = 0. Draw a graph for both. Find out the coordinates of the vertices of the triangle formed by these lines and the x-axis, and shade the triangular region.

Solution:

xy+1=0

Or

x=y1
x 0 1 2
y 1 2 3

 

3x+2y12=0x=122y3

Hence, the graphic representation is as follows.

e3.2q 7

From the figure, it can be observed that these lines are intersecting each other at point (2, 3) and x-axis at (−1, 0) and (4, 0). Therefore, the vertices of the triangle are (2, 3), (−1, 0), and (4, 0).

 

Exercise 3

 

Question 1:

Using substitution method, solve the given pair of linear equations.

  1. i) x – y = 3

x3+y2=6

  1. ii) a + b = 1

a – b = 4

iii)  0.2a + 0.3b = 1.3

0.4a+0.5b = 2.3

  1. iv) 3a – b = 3

9a – 3b = 9

  1. v) 3x25y3=2

x3+y2=136

  1. vi) 2x+3y=0

3x8y=0

Solution:

i) x – y = 3- – – – – – – – – – – (I)

x3+y2=6– – – – – – – – (II)

From (I), we get

x = y + 3- – – – – — – – – (III)

Substituting the value of x in equation (II), we get

y+33+y2=6

2y + 6 + 3y = 36

5y = 30

y = 6- – – – – – – – – (IV)

Substituting the value of y in equation (III), we get x = 9

∴ x = 9, y = 6

 

ii) a + b = 14- – – – – – – – – (I)

a – b = 4  — – – – – – – – – (II)

From (I), we get a = 14 – b- – – – – – – – – –  (III)

Substituting the value of a in equation (II), we obtain

(14 – b) – b = 4

14 – 2b = 4

10 = 2b

b = 5- – – – – – – – –  (IV)

Substituting (IV) in (III), we get

a = 14 – b

= 14 – 5

= 9

∴ a = 9, b = 5

 

iii)  0.2a + 0.3b = 1.3- – – – – – – (I)

0.4a + 0.5b = 2.3- – – – – – – –  (II)

From equation (I), we get

a=1.30.3b0.2 – – – – – – – – (III)

Substituting the value of a in equation (II), we get

0.4(1.30.3b0.2)+0.5b=2.3

2.6 – 0.6b + 0.5b = 2.3

2.6 – 2.3 = 0.1b

0.3 = 0.1b

b = 3- – – – – – – – (IV)

Substituting the value of b in equation (III), we get

a=1.30.330.2 =1.30.90.2=0.40.2=2

∴ a = 2, b = 3

 

iv) 3a – b = 3- – – – – – – – (I)

9a – 3b = 9  – – – – – – – – (II)

From (I), we get b = 3a – 3 – – – – – – – – (III)

Substituting the value of b in equation (II), we get

9a – 3(3a – 3) = 9

9a – 9a + 9 = 9

9 = 9

This is always true.

The given pair of equations has infinite solutions and the relation between these variables can be given by b = 3a – 3

Therefore, one of its possible solutions is a = 1, b = 0.

 

v) 3x25y3=2 – – – – – – – – (I)

x3+y2=136 – – – – – – – – (II)

From equation (I), we get

9x – 10y = -12

x=12+10y9 – – – – – – – – (III)

Substituting the value of x in equation (II), we get

12+10y93+y2=136 12+10y27+y2=136 24+20y+27y54=136

47y = 117 + 24

47y = 141

y = 3 – – – – – – – –  (IV)

Substituting the value of y in equation (III), we get

x=12+1039=189=2

Hence, x = 2, y = 3

 

vi) 2x+3y=0 – – – – – – – – (I)

3x8y=0 – – – – – – – – (II)

From equation (I), we get

x=3y2 – – – – – – – –  (III)

Substituting the value of x in equation (II), we get

3(3y2)8y=0 3y222y=0 y(3222)=0

y = 0- – – – – – – – (IV)

Substituting the value of y in equation (III), we get

x = 0

∴ x = 0, y = 0

 

Question 2:

Solve 2a + 3b = 11 and 2a – 4b = -24 and calculate the value of m in b = ma + 3.

Solution:

2a + 3b = 11                    (I)

2a – 4b = -24                   (II)

From equation (II), we get

a=113b2                   (III)

Substituting the value of a in equation (II), we get

2(113b2)4b=24

11 – 3b – 4b = -24

-7b = -35

b = 5                  (IV)

Putting the value of b in equation (III), we get

a=11352=42=2

Hence, a = -2, b = 5

Also,

b = ma + 3

5 = -2m +3

-2m = 2

m = -1

 

Question 3:

Using substitution method, find the solution by forming the pair of linear equations for the given problems.

i) The coach of a cricket team buys 7 bats and 6 balls. It cost her Rs 3800. Later, she buys 3 bats and 5 balls. It cost her Rs 1750. Find the cost of single bat and single

Solution:

Let the cost a bat be x and cost of a ball be y.

According to the question,

7x + 6y = 3800 …………….. (I)

3x + 5y = 1750 ……………… (II)

From (I), we get

y=38007x6 ………………. (III)

Substituting (III) in (II). we get

3x+5(38007x6)=1750 3x+9500335x6=1750 3x35x6=175095003 18x35x6=525095003 17x6=42503

-17x = -8500

x = 500 ……………….. (IV)

Substituting the value of x in (III), we get

y=380075006=3006=50

Hence, the cost of a bat is Rs 500 and cost of a ball is Rs 50.

 

ii) The difference between two numbers is 26 and one number is three times the older. Find the two numbers.

Solution:

Let the two numbers be x and y respectively, such that y > x.

According to the question,

y = 3x ……………… (1)

y – x = 26 …………..(2)

Substituting the value of (1) into (2), we get

3x – x = 26

x = 13 ……………. (3)

Substituting (3) in (1), we get y = 39

Hence, the numbers are 13 and 39.

 

iii) Five years hence, the age of Rahul will be three times that of his son. Five years ago, Rahul’s age was seven times that of his son. Calculate their present ages.

Solution:

Let the age of Rahul and his son be x and y respectively.

According to the question,

(x + 5) = 3(y + 5)

x – 3y = 10 ………………… (1)

(x – 5) = 7(y – 5)

x – 7y = -30 ……………….. (2)

From (1), we get x = 3y + 10 ……………………. (3)

Substituting the value of x in (2), we get

3y + 10 – 7y = -30

-4y = -40

y = 10 ………………… (4)

Substituting the value of y in (3), we get

x = 3 x 10 + 10

= 40

Hence, the present age of Rahul and his son is 40 years and 10 years respectively.

 

iv) If 2 is added to both the numerator and the denominator a fraction becomes 8/11. If 3 is added to both the numerator and the denominator it becomes 5/6. Find the fraction.

Solution:

Let the fraction be x/y.

According to the question,

x+2y+2=911

11x + 22 = 9y + 18

11x – 9y = -4 …………….. (1)

x+3y+3=56

6x + 18 = 5y +15

6x – 5y = -3 ………………. (2)

From (1), we get x=4+9y11 …………….. (3)

Substituting the value of x in (2), we get

6(4+9y11)5y=3

-24 + 54y – 55y = -33

-y = -9

y = 9 ………………… (4)

Substituting the value of y in (3), we get

x=4+8111=7

Hence the fraction is 7/9.

 

v) The larger of two supplementary angles exceeds the smaller by 18o. Find the two angles.

Solution:

Let the larger angle by xo and smaller angle be bo.

We know that the sum of two supplementary pair of angles is always 180o.

According to the question,

x + y = 180o……………. (1)

x – y = 18……………..(2)

From (1), we get x = 180o – y …………. (3)

Substituting (3) in (2), we get

180– y – y =18o

162o = 2y

y = 81o ………….. (4)

Using the value of y in (3), we get

x = 180o – 81o

= 99o

Hence, the angles are 99o and 81o.

 

vi) A taxi charge consists of a fixed charge + charge for the distance covered. For a distance of 10 km and 15 km, the charge paid is Rs 105 and Rs 155 respectively. Calculate the fixed charge and the charge per km. Also, calculate travel cost for 25 km.

Solution:

Let the fixed charge be Rs x and per km charge be Rs y.

According to the question,

x + 10y = 105 …………….. (1)

x + 15y = 155 …………….. (2)

From (1), we get x = 105 – 10y ………………. (3)

Substituting the value of x in (2), we get

105 – 10y + 15y = 155

5y = 50

y = 10 …………….. (4)

Putting the value of y in (3), we get

x = 105 – 10 * 10 = 5

Hence, fixed charge is Rs 5 and per km charge = Rs 10

Charge for 25 km = x + 25y = 5 + 250 = Rs 255

 

 

Exercise 4

Question 1:

Using the method of elimination and method of substitution, solve the given liner equations.

(1) a + b = 5 and 2a – 3b = 4

(2) 3a + 4b = 10 and 2a – 2b = 2

(3) 3a = 5b – 4 = 0 and 9a = 2b + 7

(4) a2+2b3=1 and ab3=3

Solution:

(1) By the method of elimination.

A + B = 5   – – – – – – – – (i)

2A – 3B = 4 – – – – – – – – (ii)

When the equation (i) and is multiplied by (ii), we get

2A + 2B = 10 – – – – – – – – (iii)

When the equation (ii) is subtracted from (iii) we get,

5B = 6

B = 65 – – – – – – – (iv)

Substituting the values obtained in (i) we get,

A=565=195 A=195,B=65

By the method of substitution:

From the equation (i), we get:

A = 5 – B – – – – – – – (v)

When the value is put in equation (ii) we get,

2(5 – B) – 3B = 4

-5B = -6

B = 65

When the values are substituted in equation (v), we get:

A=565=195 A=195,B=65

 

(2) By the method of elimination:

3A + 4B = 10 – – – – – – -(i)

2A – 2B = 2 – – – – — – – (ii)

When the equation (i) and (ii) is multiplied by 2, we get:

4A – 4B = 4 – – – – – – – (iii)

When the Equation (i) and (iii) are added, we get:

7A = 14

A=2 – – – – – – – – – (iv)

Substituting equation (iv) in (i) we get,

6 + 4B = 10

4B = 4

B = 1

Hence, A = 2 and B = 1

By the method of Substitution

From equation (ii) we get,

A = 1 + B – – – – – – – – (v)

Substituting equation (v) in equation (i) we get,

3(1 + B) + 4B = 10

7B = 7

B= 1

When B= 1 is substituted in equation (v) we get,

A = 1 + 1 = 2

Therefore, A =2 and B = 1

(3) By the method of elimination:

3A – 5B – 4 = 0 – – – – – – – (i)

9A = 2B + 7

9A – 2B – 7 = 0 – – – – – – – (ii)

When the equation (i) and (iii) is multiplied we get,

9A – 15B – 12 = 0 – – – – – (iii)

When the equation (iii) is subtracted from equation (ii) we get,

13B = -5

B=513 – – – – – – – – (iv)

When equation (iv) is substituted in equation (i) we get,

3A+25134=0 3A=2713 A=913 A=913,B=513

By the method of Substitution:

From the equation (i) we get,

A=5B+43 – – – – – – – – (v)

Putting the value (v) in equation (ii) we get,

9(5B+43)2B7=0

13B = -5

B=513

Substituting this value in equation (v) we get,

A=5(513)+43 A=913 A=913,B=513

(4) By the method of Elimination

A2=2B3=1

3A + 4B = -6 – – – – – – – (i)

AB3=3

3A – B = 9 – – – – – – – (ii)

When the equation (ii) is Subtracted from equation (i) we get,

5B = -15

B = 3 – – – – – – (iii)

When the equation (iii) is substituted in (i) we get,

3A – 12 = -6

3A = 6

A = 2

Hence, A = 2 , B = -3

By the method of Substitution:

From the equation (ii) we get,

A=B+93 – – – – – – – – – (v)

Putting the value obtained from equation (v) in equation (i) we get,

3(B+93)+4B=6

5B = -15

B= -3

When B= -3 is substituted in equation (v) we get,

A=3+93=2

Therefore, A = 2 and B = -3

 

Question 2:

Find the solutions for the given pair of linear equation by the method of elimination for the following questions:

(1) If 1 is added in the numerator and 1 subtracted from that of the denominator, a fraction is then reduced to 1. It becomes ½ if 1 is added only to the denominator. What is the fraction?

(2) Reuben was thrice as old as Alex, 5 years back. And Reuben will be twice as old as Alex, 10 years from now. What are the ages of Reuben and Alex?

(3) The sum of digits of a two digit number is 9. Also when this number is multiplied nine times, it is two times the number when obtained by reversing the order of the digits. Find the number.

(4) Manna went to the bank to withdraw a sum of Rs 2000. She had asked the cashier to give her Rs 50 and Rs 100 notes only. Manna got 25 notes in total. Find how many Rs 50 notes and Rs 100 notes she had received.

(5) A lending library has a fixed charge for the first three days and an additional charge for each day thereafter. Sara paid a sum of Rs 27 for a book to keep it for seven days, while Rosy paid a sum of Rs 21 for the book that she had kept for five days. Find the fixed charge and the charge that will cost for each extra day.

Solution:

(1) Let the fraction be a/b

According to that of the given information,

A+1B1=1 => A – B = -2 – – – – – – – – – – – (i)

AB+1=12 => 2A – B = 1 – – – – – – – – (ii)

When equation (i) is Subtracted from equation (ii) we get A = 3 – – – – – – (iii)

When A = 3 is substituted in equation (i) we get,

3 – B = -2

-B = -5

B = 5

Hence, the fraction is 3/5

 

(2) Let the present age of Reuben = a

And the present age of Alex = b

According to the information that is given,

(A – 5 ) = 3 ( B – 5 )

A – 3B = -10 – – – – – – – – – (i)

(A + 10) = 2(B + 10)

A – 2B = 10 – – – – – – – – – – – (ii)

When the equation (i) is subtracted from equation (ii) we get,

B = 20 – – – – – – – – – (iii)

Substituting B = 20 in equation (i), we get:

A – 60 = -10

A = 50

Hence, the present age of Reuben is 50 yrs

And the present age of Alex is 20 yrs

 

(3) Let the unit digit and tens digit of a number be A and B respectively.

Then, Number (n) = 10B + A

N after reversing the digits = 10A + B

According to the given information, A + B = 9 – – – – – – – – – (i)

9(10B + A) = 2(10A + B)

88 B – 11 A = 0

-A + 8B = 0 – – – – – – – – (ii)

Adding the equations (i) and (ii) we get,

9B = 9B

= 1 (3)

Substituting this value in the equation (i) we get A= 8

Hence the number (N) is 10B + A = 10 x 1 +8 = 18

 

(4) Let the number of Rs 50 notes be A and the number of Rs 100 notes be B

According to the given information,

A + B = 25 – – – – – – – – – – – (i)

50A + 100B = 2000 – – – – – – – (ii)

When equation (i) is multiplied with (ii) we get,

50A + 50B = 1250 – – – – – – – – – (iii)

Subtracting the equation (iii) from the equation (ii) we get,

50B = 750

B = 15

Substituting in the equation (i) we get,

A = 10

Hence, Manna has 10 notes of Rs 50 and 15 notes of Rs 100.

 

(5) Let the fixed charge for the first three days be Rs A and the charge for each day extra be Rs B.

According to the information given,

A + 4B = 27 – – – – – (i)

A + 2B = 21 – – – – – – (ii)

When equation (ii) is subtracted from equation (i) we get,

2B = 6

B = 3 – – – – – – (iii)

Substituting B = 3 in equation (i) we get,

A + 12 = 27

A = 15

Hence, the fixed charge is Rs 15

And the Charge per day is Rs 3

 

  Exercise-5

Question 1:

 From the given pairs of linear equations find the ones with unique solution, no solution or infinitely many solutions. If there is a unique solution, find it by using cross multiplication method.

(i) x-3y-3 =0 and  3x-9y-2=0               (ii) 2x+y =5 and 3x +2y =8

(iii)6x – 10y =40 and 3x-5y = 20    (iv)x -3y – 7 =0  and 3x -3y -15=0

 

Solution:

(i)      3x – 9y -2 =0

x – 3y – 3 =0

a1/a1=1/3 ,         b1/b2  = -3/-9 =1/3,     c1/c2=-3/-2 = 3/2

a1a2=b1b2c1c2

Since, the given set of lines are parallel to each other they will not intersect each other and     therefore there is no solution for these equation.

 

(ii) 2x + y = 5

3x +2y = 8

a1a2=23,b1b2=12,c1c2=58 a1a2b1b2

Since they intersect at a  unique point these equations will have a unique solution by cross multiplication method:

xb1c2c1b2=yc1a2c2a1=1a1b2b1a2 x8+10=y15+16=143

x/ 2 = y/1 = 1

x =2 , y =1.

 

(iii) 6x -10y = 40

3x – 5y = 20

a/ a2 = 3/6 = 1/2 ,               b1/b2 = -5/-10 =1/2,     c1/c2 =-20/-40 = ½

a1/a= b1/b2 = c1/c2

Since the given sets of lines are overlapping each other there will be infinite number of solutions for this pair of equation.

 

(iv) 3x – 3y – 15 =0

x – 3y – 7 = 0

a1/a= 1/3,     b1/b2 = 1 ,   c1/c= -7/-15

a1a2b1b2

Since this pair of lines are intersecting each other at a unique point, there will be  a unique solution.

By cross multiplication,

x/(45-21) = y/(-21+15) = 1/(-3+9))

x/24 = y/-6 =1/6

x/24 = 1/6 and y/-6 = 1/6

∴  x = 4 and y =1.

 

Question 2:

(i) Find the values of a and b at which the following pair of linear equations will have infinite solution.

3y + 2x = 7

(a+b)y +(a-b)x = 3a + b -2

(ii) Find the value of k at which the following set of linear equations won’t have a solution.

3x + y =1

(2k – 1)x + (k-1)y = 2k + 1

Solution:

(i) 3y + 2x -7 =0

(a + b) y + (a-b)y – (3a + b -2) = 0

a1/a2=2/a-b ,               b1/b2=3/a+b ,               c1/c2=-7/-(3a + b -2)

For infinitely many solutions,

a1/a= b1/b= c1/c2

2/a-b = 7/3a + b – 2

6a + 2b – 4 = 7a – 7b

a – 9b = -4  ……(i)

2/a-b = 3/a+b

2a + 2b = 3a – 3b

a -5b = 0 …….(ii)

Subtracting (i) from (ii), we get

4b = 4

b =1

Substituting this eqn in (ii), we get

a -5 x 1= 0

a = 5

Thus at a = 5 and b = 1 the given equations will have infinite solutions.

 

(ii) 3x + y -1 = 0

(2k -1)x  +  (k-1)y – 2k -1 = 0

a1/a2 = 3/2k -1 ,           b1/b2=1/k-1 ,c1/c2 = -1/-2k -1 = 1/ 2k +1

For no solutions

a1/a2  = b1/b2 ≠ c1/c2

3/2k-1 = 1/k -1   ≠ 1/2k +1

3/2k – 1 = 1/k -1

3k -3 = 2k -1

k =2

Therefore, for k = 2 the given pair of linear equations will have no solution.

 

Question 3:

Solve the given pair of linear equations using cross multiplication method and substitution method:

3x + 2y = 4

8x + 5y =9

Solution:

  1. 8x + 5y = 9 ….(1)

3x   +  2y = 4 ….(2)

From equation (2) we get

x = 4 – 2y / 3  …. (3)

Using this value in equation 1 we get

8(4-2y)/3 + 5y = 9

32 – 16y +15y = 27

-y = -5

y = 5 ……(4)

Using this value in equation (2), we get

3x + 10 = 4

x = -2

Thus , x = -2 and y = 5.

 

Now, Using Cross Multiplication method:

8x +5y – 9 = 0

3x + 2y – 4 = 0

x/(-20+18) = y/(-27 + 32 ) = 1/(16-15)

-x/2 = y/5 =1/1

Thus , x = -2 and y =5.

 

Question 4:

For the problems given below, form a pair of linear equations and also solve them (if a solution exists).

(i) When 1 is subtracted from the numerator of a fraction it becomes 1/3 and it becomes 1/4 when 8 is added to the denominator. Find the fraction.

(ii) Gah and Hag are 100kms apart on a highway. A car starts from Gah and another from Hag at the same time. If the cars move towards each other, they meet in one hour and if they move in the same direction, they meet after 5 hours. Find the speed of the two cars.

(iii) Monthly hostel charges are fixed, except for the mess charge which is included in it. Dom a eats for 20 days in the mess and pays Rs1000 as her monthly hostel charge. Nimi eats for 26 days and pays Rs.1180 as hostel charges. What is the fixed charge and the cost of food per day?

(iv) Lama scored 40 in an exam, acquiring 3 marks higher for each right answer and losing 1 mark for every wrong answer. If 4 marks had been awarded for every correct answer and 2 marks reduced for every wrong answer, Lama would have scored 50 marks. Find the number of questions in the exam.

Solution:

(i)    let the fraction be x/y

So, according to question :

(x-1)/y = 1/3 ­­­ =>  3x – y = 3…..(1)

x/(y + 8) = 1/4  => 4x –y =8 ….(2)

Subtracting equation (1) from (2) , we get

x = 5 ……(3)

Using this value in equation (2) we get

4×5 – y = 8

y= 12.

Therefore, the fraction is 5/12.

 

(ii)Let the speed of the first and second cars be x km/h and y km/h respectively.

Respective speed of the two cars when they are moving in the same direction = (x -y) km/h

Respective speed of the two cars when they are headed towards each other = (x + y)km/h

According to the question;

5(x-y) =100

    x – y =20 ….. (1)

Also,

1(x +y )=100 …..(2)

Adding equations (1) and (2) we get

2x =120

x = 60 km/h ……(3)

Using this in equation (1) we get

60-20= y

y= 40 km/h.

 

(iii) let x be the fixed charge and y be the charge of food per day.

According to the question

x + 20y = 1000…. (i)

x +  26y = 1180….(ii)

Subtracting (i) from  (ii) we get

6y=180

y= Rs.30

Using this value in equation (ii) we get

x = 1180 -26 x 30

= Rs.400.

 

(iv) Let x be the number of correct numbers and y be the number of incorrect answers.

According to the question,

3x – y = 40 …..(i)

4x – 2y = 50

    2x-y = 25….(ii)

Subtracting equation (ii) from (i), we get

x=15…………(iii)

Using this in equation (i) we get

3(15) – 40 =y

y= 5

Therefore, the number of correct answers = 15

the number of incorrect answers= 5

the number of questions              =20

 

  EXERCISE – 6

 

Solve the following pairs of equations by reducing them to a pair of linear equations:

Question 1:

 12a+13b=2

 13a+12b=136

 Solution:

 Let us assume 1a=mand1b=n, then the equation will change as follows.

m2+n3=23m+2n12=0   …….(1)

m3+n2=1362m+3n13=0   …….(2)

Now, Using cross-multiplication method, we get

m26(36)=n24(39)=194 m10=q15=15 m10=15andq15=15

m = 2 and n = 3

1a=2and1b=3 a=12andb=13

 

Question 2:

4a+3b=14

4a4b=23

 

Solution:

 Putting 1a=m in the given equation we get,

So, 4m + 3b = 14     => 4m + 3b – 14 = 0  ………..(1)

3m – 4b = 23     => 3m – 4b – 23 = 0  ………..(2)

By cross-multiplication, we get,

m6956=b42(92)=1169 m125=b50=125 m125=125andb50=125

m = 5 and b = -2

m = 1a = 5

a = 15

b = -2

 

Question 3:

7a2bab=5

7b2a=5 …..(i)

8a+7bab=15

8b+7a=15 …..(ii)

Solution:

 Substituting 1a=m in the given equation we get,

– 2m + 7n = 5     => -2 + 7n – 5 = 0  ……..(iii)

7m + 8n = 15     => 7m + 8n – 15 = 0 ……(iv)

By cross-multiplication method, we get,

m105(40)=n3530=11649 m65=n65=165 m65=165andb65=165

m = 1 and n = 1

m = 1a = 1        n = 1a = 1

a = 1    b = 1

 

Question 4:

10a+b+2ab=4

15a+b5ab=2

Solution:

Substituting 1a+b=m and 1ab=n in the given equations, we get,

10m + 2n = 4      =>  10m + 2n – 4 = 0      …..(i)

15m – 5n = -2     =>   15m – 5n + 2 = 0    …..(ii)

Using cross-multiplication method, we get,

m420=n60(20)=15030 m16=n80=180 m16=180andb80=180

m = 15 and n = 1

m = 1a+b=15 and n = 1ab=1

a + b = 5  …..(iii)

and a – b = 1 …..(iv)

Adding equation (iii) and (iv), we get

2a = 6   => a = 3 …….(v)

Putting the value of a = 3 in equation (3), we get

b = 2

Hence, a = 3 and b = 2

 

Question 5:

2a+3b=2

4a9b=1

 

Solution:

 Substituting 1a=m and 1b=n in the given equations, we get

2m + 3n = 2 …….(i)

4m – 9n = -1 ……(ii)

Multiplying equation (i) by 3, we get

6m + 9n = 6 ………..(iii)

Adding equation (ii) and (iii), we get

10m = 5

m = 12 ……(iv)

Now by putting the value of ‘m’ in equation (i), we get

2×12+2n=2

3n = 1

n = 13

m = 1a=12

a=2

a = 4

n = 1b=13

b=3

b = 9

Hence, a = 4 and b = 9

 

Question 6:

5a1+1b2=2

     6a13b2=1

 

Solution:

 Substituting 1a1=m and 1b2=n in the given equations, we get,

5m + n = 2 ……(i)

6m – 3n = 1 …..(ii)

Multiplying equation (i) by 3, we get

15m + 3n = 6 ……..(iii)

Adding (ii) and (iii) we get

21m = 7

m = 13

Putting this value in equation (i), we get

5×13+n=2 n=253=13 m=1a1=13a1=3a=4 n=1b2=13b2=3b=5

Hence, a = 4 and b = 5

 

Question 7:

6a + 3b = 6ab

=> 6b+3a=6 …..(i)

    2a + 4b = 5ab

=> 2b+4a=5 …….(ii)

Substituting 1a=m and 1b=n
3m + 6n – 6 = 0

4m + 2n – 5 = 0

 

By cross-multiplication method, we get

m30(12)=n24(15)=1624 m18=n9=118 m18=118andn9=118

m = 1 and n = 1/2

m = 1/a = 1 and n = 1/b = 1/2

a = 1                              b = 2

Hence, a = 1 and b = 2

 

Question 8:

13a+b+13ab=34

     12(3a+b)1